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Failure Modes, Effects and Criticality Analysis and Accelerated Life testing of
LEDs for Medical applications
M. Sawant and A. Christou University of Maryland
College Park MD.
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Agenda
1. Medical Diagnostic Application for LEDs 2. Failure Modes and Effects Criticality Analysis (FMECA) - Failure Modes & Mechanisms of AlGaInP/GaP LEDs - Initial FMECA Analysis (Before Accelerated Life Test) - Residual FMECA Analysis (After Accelerated Life Test) 3. Accelerated Life Testing (ALT) of AlGaInP/GaP LEDs - Inverse Power Law Model for High Current & Arrhenius Model for High Temperature Aging - Logarithmic Degradation Rate modeling - Optical Power vs. Vf performance during ALT - Peak wavelength & Full Width Half Max (FWHM) during ALT 4. Regression Analysis of Prior Publ.Data for AlGaInP/GaP LEDs 5. Conclusions.
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Medical Diagnostic Application for LEDs
LEDLightsource
HumanEyeObject
Lighting Appl. Fiber Optics Appl.
LEDLightsource
Lens +Filter
Splice SignalReceiver
OpticalFiber
Fig. 1.1a LED in Lighting / Fiber Optics Application
LEDLightsource
Lens +Filter
BeamSplitter
TransparentCuvette + Sample
SignalReceiver
ReferenceReceiver(Optional)
Fig. 1.1b LED in Medical Diagnostic Application Interpret Patient Results
20% degradation means LED
failure!
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Generic AlGaInP/GaP LED structure & fab. process
LED epitaxial structure grown on GaAs substrate
Diagram not to scale
AlGaInP
GaAs
p-GaP
GaAs substrate chemically removed
AlGaInP p-GaP
n-GaP wafer bonded in place of GaAs substrate
AlGaInP
n-GaP
p-GaP
High Temperature & Pressure
Contacts added & packaging
AlGaInP
n-GaP
p-GaP
AlGaInP/GaP chip fabrication process, Vanderwater et al [6]
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Failure Modes/Mechanisms AlGaInP/GaP LEDs
AlGaInP
p-GaP
n-GaP
P - N Metal contacts [2]: - Interdiffusion
ITO layer [16] for current spreading & light extraction: - Loss of Oxygen from ITO - De-adhesion
Plastic Encapsulation [21]: - Discoloration - Carbonization - Polymer degradation
Packaging [2] failure: Heat sink - Heat sink de-lamination
Active Region [6,7,12]: - Dislocation growth - Metal diffusion in AlGaInP - Heating effects of AlGaInP active region resulting in enhanced current injection
Packaging failure: Bond Wires [2] - Electro-migration of bond wires - Burnout due to excessive current - Void formation at the solder metal stem - Reaction of solder metal with package electrodes
Photon Current
Diagram not to scale
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Failure Modes & Effects Criticality Analysis
Severity Classification for Medical Diagnostic Application Level Rating Description for Medical Diagnostic InstrumentCatostrophic 1 Inaccurate analytical result,
Erroneous But Believable (EBB) resultMay lead to death of patient or user or Serious deterioration in their state of health
Critical 2 Incorrect diagnosis or use of less effective or inappropriate treatment
Marginal 3 Possible Erroneous But Believable (EBB) result, Test is used in conjunction with other diagnostic information.
Minor 4 Delayed or no medical test result, Incorrect result causing no difference in diagnosis or no inappropriate treatment, Incorrect result requiring reflex or confirmatory testing.
None 5 ---
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FMECA continued
Occurrence Classification Level Rating DescriptionFrequent 1 A single failure mode probability > 20% of overall component failure probability
Resonably Probable 2A single failure mode probability > 10% and < 20%of overall component failure probability
Occassional 3A single failure mode probability > 1% and < 10%of overall component failure probability
Remote 4A single failure mode probability > 0.1% and < 1%of overall component failure probability
Extremely Unlikely 5 A single failure mode probability < 0.1%of overall component failure probability
Failure mode criticality number Cm=βαλT β – Failure Effect Probability α – Failure Mode Ratio λ – Failure Rate T – Operating Time
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FMECA Initial (Before Accelerated Life Test) Sr.# Failure
Modes/Mechanisms
Causes Local Effects at LED level
System Effects in Medical equipment
Severity
Failure Effect Probability (ß)
Failure Mode Ratio (a)
Failure Rate
Operating Time (T)
in hrs
Criticality #
1 Packaging failure (Heat Sink)
Heat sink de-lamination - Decrease of optical output- Local heating effects
- Unscheduled module replacement- Delayed medical test results
3 0.4 0.3 1.8E-11 31500 6.7E-08
2 Degradation of plastic encapsulation
- Discoloration- Carbonization- Polymer degradation at high temperature
- Gradual decrease of optical output
- Excessive drift requires unscheduled calibration- Delayed medical test results
3 0.4 0.2 1.8E-11 31500 4.5E-08
3 Degradation of ITO layer
- Loss of Oxygen from ITO- De-adhesion
- Decrease of optical output- Non-uniform light emission
- Unscheduled module replacement- Delayed medical test results
4 0.3 0.1 1.8E-11 31500 1.7E-08
4 Packaging failure (Bond Wires)
- Electro-migration of bond wires- Burnout due to excessive current- Void formation at the solder metal stem- Reaction of solder metal with package electrodes
- Abrupt LED failure
- Unscheduled module replacement- Delayed medical test results
4 0.9 0.1 1.8E-11 31500 5.0E-08
5 Degradation of active layer
- Dislocation growth- Metal diffusion in AlGaInP- Heating effects of AlGaInP active region resulting in enhanced current injection
- Gradual decrease of optical output
- Excessive drift requires unscheduled calibration- Delayed medical test results
4 0.4 0.4 1.8E-11 31500 9.0E-08
6 Degradation of P-N metal contacts
- Interdiffusion - Change in IV characteristics
- Design will accommodate minor changes in IV characteristics
5 0.4 0.2 1.8E-11 31500 4.5E-08
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Accelerated Life Test Setup
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Setup for LED Characterization
Fig. 2.3.3 Setup for LED Characterization
Current Drive
Integrating Sphere
Feedback for IV Characteristics Control &
Timing
Irradiance Fiber
PC + Test
Software
Spectro- Radiometer
Signal Conditioning
LED Data Acquisition
Board Trigger Pulses
LED Driver Circuit
V, I values
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Accelerated Life Test Conditions
Test Conditions: Burst/Pulse Testing: Pulse On: 0.1ms Pulse Off: 19.9ms # of Pulses: 100 Burst On: 2s Burst Off: 3s Burst Period: 5s Duty Cycle: 100 x 0.1/5000 = 0.2%
LED Driver: Separate driver for each of the 15 LEDs Pulsed at constant Peak Current: 483mA for Batch 2 Pulsed at constant Peak Current: 725mA for Batch 3
Chamber 1: 35°C 5 LEDs in each Batch
Chamber 2: 55°C 5 LEDs in each Batch
Chamber 3: 75°C 5 LEDs in each Batch
2sec 3sec
Burst 1: 5sec
2sec 3sec
Burst 2: 5sec
Continues 24/7 until interrupted for regular Optical Measurements
1 2 100 3... 1 2 100 3...
1 2 100 3... 1 2 100 3...
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Inverse Power Law Modeling: Current Density The inverse power law relation ship is given as
nJATTF −= . - (1)
Where TTF=Time to failure in hrs, J=LED Current density in Amps/sq2, A &n are +ve constants
Taking Ln on both sides,
JnLnLnATTFLn .)( −= - (2)
This gives a straight line relationship where ‘-n’ is the slope, LnJ is theaccelerating variable and LnA is the y-intercept. The negative slope impliesthat as the current density increases, the TTF decreases.
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Arrhenius Reaction Rate Modeling: Temperature
Arrhenius reaction rate model is
−
= KTEa
BeRate - (3)
Where T=Temperature in °K, Ea=Activation energy of the LEDdegradation, K=Boltzmann’s constant, B is another constant.
Taking reciprocal of the ‘rate’ to get ‘time’
= KTEa
CeTTF - (4)
Where TTF=Time to failure in hrs, C=1/B is another constant. Taking Ln,
KTEaLnCTTFLn +=)( - (5)
A straight line relationship where ‘Ea/K’ as slope and LnC is the y-intercept.For graphing convenience, we use ‘Ea’ as slope and 1/KT as theaccelerating variable. As temperature increases, 1/KT decreases and the TTFalso decreases.
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Acceleration Factor Computation Acceleration Factor for Inverse Power Law Model is given by
n
Use
Acc
Acc
Use
JJ
TTFTTFAF
==1 - (6)
Acceleration Factor for Arrhenius Reaction Rate Model is given by
−
== AccUse TTKEa
Acc
Use eTTFTTFAF
11
2 - (7)Assume Ea & convert all TTF data to use Temperature T to estimate ‘n’Take n above & convert all TTF data to use current density J to estimate ‘Ea’.Repeat using Iterative Regression Analysis to estimate ‘Ea’ and ‘n’.
Overall Acceleration Factor is given by
−
== AccUse TTK
Ean
Use
Acc eJJxAFAFAF
11
21 - (8)
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Accelerated Life Test Data - Batch2
Test Results: Using regression, Act Energy: 1.14eV, IPL n: 4.48 Failure Modes: Active region & encapsulation, Degradation Rate: Logarithmic with Time
Peak LED current: 483mA Chamb1: 35°C Chamb2: 55°C Chamb3: 75°C Reference LED
Spectrometer Characterization 640nm LEDs
0
20
40
60
80
100
120
0 100 200 300 400 500 600
Total Time in hrs (On+Off)
Opt
ical
Pow
er R
atio
ove
r 10n
m
(Cur
rent
Po
/ Ini
tial P
i)
LED01_Avg_10nm
LED02_Avg_10nm
LED03_Avg_10nm
LED04_Avg_10nm
LED05_Avg_10nm
LED06_Avg_10nm
LED07_Avg_10nm
LED08_Avg_10nm
LED09_Avg_10nm
LED10_Avg_10nm
LED11_Avg_10nm
LED12_Avg_10nm
LED13_Avg_10nm
LED14_Avg_10nm
LED15_Avg_10nm
LED16_Avg_10nm
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Accelerated Life Test Data - Batch3
Test Results: Using regression, Act Energy: 1.14eV, IPL n: 4.48 Failure Modes: Active region & encapsulation, Degradation Rate: Logarithmic with Time
Peak LED current: 725mA Chamb1: 35°C Chamb2: 55°C Chamb3: 75°C Reference LED
Spectrometer Characterization 640nm LEDs
0
20
40
60
80
100
120
0 50 100 150 200 250 300 350
Total Time in hrs (On+Off)
Opt
ical
Pow
er R
atio
ove
r 10n
m
(Cur
rent
Po
/ Ini
tial P
i)
LED01_Avg_10nm
LED02_Avg_10nm
LED03_Avg_10nm
LED04_Avg_10nm
LED05_Avg_10nm
LED06_Avg_10nm
LED07_Avg_10nm
LED08_Avg_10nm
LED09_Avg_10nm
LED10_Avg_10nm
LED11_Avg_10nm
LED12_Avg_10nm
LED13_Avg_10nm
LED14_Avg_10nm
LED15_Avg_10nm
LED16_Avg_10nm
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Logarithmic degradation: Time To Failure Prediction
Logarithmic degradation rate of LED. In agreement with Yanagisawa et al [31]
Spectrometer Characterization 640nm LEDs
y = -1.1962Ln(x) + 93.075R2 = 0.9523
0
20
40
60
80
100
0 50 100 150 200 250 300 350
Total Time in hrs (On+Off)
Opt
ical
Pow
er R
atio
ove
r 10n
m
(Cur
rent
Po
/ Ini
tial P
i)
LED04_Avg_10nm
Log.(LED04_Avg_10nm)
Estimation of Time To Failure:Degradation Equation isy = -1.1962Ln(x) + 93.075Failure occurs when y = Po/Pi =80%80 = -1.1962Ln(x) + 93.075Ln(x) = (93.075 - 80) / 1.1962 = 10.93Time to Failure = x = Ln-1 (10.93) Time to Failure = 5.58 104 hrs
Failure threshold = 80% (for 20% degradation)
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Accelerated Life Test: LED Photos
640x1-26 On at 40µA
Clear Lens of New LED Minor Lens
degradation 640x1-21
Severe Lens degradation 640x1-34
Moderate Lens degradation 640x1-26
New Chip (340x340µm) On at 40µA
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Chip (Vf) Vs LENs degradation
Note: Vf peak measured while keeping If peak constant
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Accelerated Life Test: Spectrum change
Initial Spectrometer Characterization 640nm LEDs Batch2
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
590 600 610 620 630 640 650 660 670 680 690
Wavelength nm
Irrad
ianc
e uW
LED01_BL01LED02_BL01LED03_BL01LED04_BL01LED05_BL01LED06_BL01LED07_BL01LED08_BL01LED09_BL01LED10_BL01LED11_BL01LED12_BL01LED13_BL01LED14_BL01LED15_BL01LED16_BL01
Final Spectrometer Characterization 640nm LEDs Batch2
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
590 600 610 620 630 640 650 660 670 680 690
Wavelength nmIrr
adia
nce
uW
LED01_BL01LED02_BL01LED03_BL01LED04_BL01LED05_BL01LED06_BL01LED07_BL01LED08_BL01LED09_BL01LED10_BL01LED11_BL01LED12_BL01LED13_BL01LED14_BL01LED15_BL01LED16_BL01
Before Accelerated Life Test After Accelerated Life Test
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Accelerated Life Test: Peak Shift & narrow FWHM
For LED 640x1-26 Before Acc. Life Test λpeak = 645nm HWFM = 32.3nm
After Acc. Life Test λpeak = 643nm HWFM = 28.4nm Peak shifted to left FWHM narrowed
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Results of Accelerated Life Test Batch2
UUTTemp
C
TTF hrs Observed 20% degrd MTTF
AccFactor
Act. EnergyeV
TTF hrs Estimated 20% degrd
Equation for Logarithmic degradation model
LENs degradation
Vf Increase %
% Drop @ 630nm rel to 640nm
% Drop @ 650nm rel to 640nm
640x1-21 35 Suspend 3457.9 1755.6y = -1.4537Ln(x) + 90.86R2 = 0.8617
MinorSurface 4.0 3.2 -0.2
640x1-22 35 Suspend 335.9y = -2.8548Ln(x) + 96.606R2 = 0.9119
MinorFew Bubbles 4.6 4.4 0.4
640x1-23 35 Suspend 8282.1y = -1.4323Ln(x) + 92.922R2 = 0.9367
MinorSurface 6.5 4.6 1.0
640x1-24 35 Suspend 55851.2y = -1.1962Ln(x) + 93.075R2 = 0.9523
ModerateEdge of globe 7.1 -> 5.2 4.7 1.0
640x1-25 35 Suspend 1.8E+08y = -0.8057Ln(x) + 95.326R2 = 0.8927
ModerateSurface 4.9 3.5 2.6
640x1-26 55 207.9 201.9 17.1 1.24 195.0y = -1.5697Ln(x) + 88.277R2 = 0.9025
ModerateSurface 2.9 5.2 9.6
640x1-27 55 1.5 0.3y = -4.3842Ln(x) + 73.933R2 = 0.9732
ModerateSurface 7.9 -> 6.4 4.9 2.1
640x1-28 55 41.7 38.7y = -1.9411Ln(x) + 87.094R2 = 0.9864
MinorFew Bubbles 7.5 5.6 0.8
640x1-29 55 Suspend 1225.0y = -1.4315Ln(x) + 90.179R2 = 0.9535
MinorFew Bubbles 2.5 3.8 12.5
640x1-30 55 Suspend 9.7E+17y = -0.4285Ln(x) + 97.745R2 = 0.5402
MinorFew Bubbles 4.5 0.3 0.0
640x1-31 75 4.4 48.8 70.8 0.98 1.5y = -3.2403Ln(x) + 81.238R2 = 0.9684
ModerateSurface+Bubles 3.6 6.0 2.6
640x1-32 75 4.4 3.2y = -2.748Ln(x) + 83.18R2 = 0.9754
MinorFew Bubbles 11.7 -> 7.0 6.1 3.2
640x1-33 75 85.3 125.9y = -1.8618Ln(x) + 89.002R2 = 0.9464
Minor Very Few Bubbles 8.8 4.1 2.3
640x1-34 75 1.5 0.2y = -4.6769Ln(x) + 71.53R2 = 0.9691
SevereSurface 8.6 -> 4.5 4.6 3.5
640x1-35 75 85.3 113.4y = -1.9047Ln(x) + 89.011R2 = 0.9633
MinorFew Bubbles 11.5 -> 7.8 4.2 4.1
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Results of Accelerated Life Test Batch3
UUTTemp
C
TTF hrs Observed 20% degrd MTTF
AccFactor
Act. EnergyeV
TTF hrs Estimated 20% degrd
Equation for Logarithmic degradation model
LENs degradation
Vf Increase %
% Drop @ 630nm rel to 640nm
% Drop @ 650nm rel to 640nm
640x1-41 35 Suspend 175.2 9.7E+16y = -0.5742Ln(x) + 102.46R2 = 0.7083
ModerateSurface -0.8 1.1 1.5
640x1-42 35 Suspend 4.7E+22y = -0.3873Ln(x) + 100.22R2 = 0.7377
MinorSurface -1.6 1.8 4.4
640x1-43 35 208.1 316.3y = -2.339Ln(x) + 93.465R2 = 0.9704
ModerateBubles 8.6 5.0 -0.8
640x1-44 35 35.3 34.1y = -2.5448Ln(x) + 88.985R2 = 0.8966
MinorSurface 5.6 5.7 0.0
640x1-45 35 Suspend 6.8E+19y = -0.4472Ln(x) + 100.42R2 = 0.7487
SevereSurface Scratch 0.3 2.0 3.4
640x1-46 55 Suspend 6.0 29.2 1.47 1.2E+16y = -0.6064Ln(x) + 102.43R2 = 0.8948
ModerateSurface -0.9 0.9 2.7
640x1-47 55 Suspend 4.0E+11y = -0.7681Ln(x) + 100.51R2 = 0.9481
MinorSurface -1.6 1.2 1.7
640x1-48 55 Suspend 1.6E+14y = -0.6449Ln(x) + 101.1R2 = 0.8551
ModerateSurface -1.6 1.2 1.6
640x1-49 55 7.8 6.0y = -2.7834Ln(x) + 84.99R2 = 0.9781
MinorSurface 7.5 6.6 0.0
640x1-50 55 Suspend 3.9E+13y = -0.7512Ln(x) + 103.5R2 = 0.8257
MinorSurface -2.2 1.5 -0.9
640x1-51 75 4.4 3.7 47.9 0.89 3.8y = -2.8983Ln(x) + 83.881R2 = 0.9886
MinorSurface 7.5 6.8 0.1
640x1-52 75 0.3 0.1y = -5.7886Ln(x) + 68.472R2 = 0.986
MinorSurface 6.6 6.5 0.0
640x1-53 75 Suspend 3.9E+17y = -0.5841Ln(x) + 103.66R2 = 0.9172 None -1.6 1.0 2.7
640x1-54 75 Suspend 3.4E+17y = -0.5618Ln(x) + 102.68R2 = 0.9082
MinorSurface -2.3 0.2 1.9
640x1-55 75 7.8 7.0y = -2.7536Ln(x) + 85.365R2 = 0.9692
MinorSurface 10.1 6.2 0.5
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Regression Analysis of Published Data: AlGaInP Current Density: Inverse Power Law Temperature: Arrhenius Reaction Rate
DH-DC IPL n=1.88 Arrhen. Act.En. =0.88eV
MQW-DC IPL n=1.72 Arrhen. Act.En. =0.65eV
Effect of Current Acceleration
y = -1.7243x + 13.72
0.0
5.0
10.0
15.0
20.0
25.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Ln J
Ln o
f Life
at T
arge
t Tem
pera
ture
Ln LifexAF_TempLinear (Ln LifexAF_Temp)
Effect of Temperature Acceleration
y = 0.6549x - 13.845
0.0
5.0
10.0
15.0
20.0
25.0
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00
1/KT
Ln o
f Life
at T
arge
t Cur
rent
den
sity
Ln LifexAF_CurrentLinear (Ln LifexAF_Current)
Effect of Current Acceleration
y = -1.8791x + 14.619
0.0
5.0
10.0
15.0
20.0
25.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Ln J
Ln o
f Life
at T
arge
t Tem
pera
ture
Ln LifexAF_TempLinear (Ln LifexAF_Temp)
Effect of Temperature Acceleration
y = 0.8833x - 19.855
0.0
5.0
10.0
15.0
20.0
25.0
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00
1/KT
Ln o
f Life
at T
arge
t Cur
rent
den
sity
Ln LifexAF_CurrentLinear (Ln LifexAF_Current)
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Regression Analysis of Published Data: GaN Current Density: Inverse Power Law Temperature: Arrhenius Reaction Rate
DH-DC IPL n=2.69 Arrhen. Act.En. =0.49eV
MQW-DC IPL n=2.00 Arrhen. Act.En. =0.51eV
Effect of Current Acceleration
y = -2.6944x + 17.388
0.0
5.0
10.0
15.0
20.0
25.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Ln J
Ln o
f Life
at T
arge
t Tem
pera
ture
Ln LifexAF_TempLinear (Ln LifexAF_Temp)
Effect of Temperature Acceleration
y = 0.499x - 5.7632
0.0
5.0
10.0
15.0
20.0
25.0
36.50 37.00 37.50 38.00 38.50 39.00 39.50
1/KT
Ln o
f Life
at T
arge
t Cur
rent
den
sity
Ln LifexAF_CurrentLinear (Ln LifexAF_Current)
Effect of Current Acceleration
y = -2.0003x + 14.152
0.0
5.0
10.0
15.0
20.0
25.0
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Ln J
Ln o
f Life
at T
arge
t Tem
pera
ture
Ln LifexAF_TempLinear (Ln LifexAF_Temp)
Effect of Temperature Acceleration
y = 0.5153x - 7.7551
0.0
5.0
10.0
15.0
20.0
25.0
0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00
1/KT
Ln o
f Life
at T
arge
t Cur
rent
den
sity
Ln LifexAF_CurrentLinear (Ln LifexAF_Current)
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FMECA Initial (Before Accelerated Life Test) Sr.# Failure
Modes/Mechanisms
Causes Local Effects at LED level
System Effects in Medical equipment
Severity
Failure Effect Probability (ß)
Failure Mode Ratio (a)
Failure Rate
Operating Time (T)
in hrs
Criticality #
1 Packaging failure (Heat Sink)
Heat sink de-lamination - Decrease of optical output- Local heating effects
- Unscheduled module replacement- Delayed medical test results
3 0.4 0.3 1.8E-11 31500 6.7E-08
2 Degradation of plastic encapsulation
- Discoloration- Carbonization- Polymer degradation at high temperature
- Gradual decrease of optical output
- Excessive drift requires unscheduled calibration- Delayed medical test results
3 0.4 0.2 1.8E-11 31500 4.5E-08
3 Degradation of ITO layer
- Loss of Oxygen from ITO- De-adhesion
- Decrease of optical output- Non-uniform light emission
- Unscheduled module replacement- Delayed medical test results
4 0.3 0.1 1.8E-11 31500 1.7E-08
4 Packaging failure (Bond Wires)
- Electro-migration of bond wires- Burnout due to excessive current- Void formation at the solder metal stem- Reaction of solder metal with package electrodes
- Abrupt LED failure
- Unscheduled module replacement- Delayed medical test results
4 0.9 0.1 1.8E-11 31500 5.0E-08
5 Degradation of active layer
- Dislocation growth- Metal diffusion in AlGaInP- Heating effects of AlGaInP active region resulting in enhanced current injection
- Gradual decrease of optical output
- Excessive drift requires unscheduled calibration- Delayed medical test results
4 0.4 0.4 1.8E-11 31500 9.0E-08
6 Degradation of P-N metal contacts
- Interdiffusion - Change in IV characteristics
- Design will accommodate minor changes in IV characteristics
5 0.4 0.2 1.8E-11 31500 4.5E-08
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FMECA Residual (After Accelerated Life Test) Sr.# Failure
Modes/Mechanisms
Causes Local Effects at LED level
System Effects in Medical equipment
Severity
Failure Effect Probability (ß)
Failure Mode Ratio (a)
Failure Rate
Operating Time (T)
in hrs
Criticality #
1 Packaging failure (Heat Sink)
Heat sink de-lamination - Decrease of optical output- Local heating effects
- Unscheduled module replacement- Delayed medical test results
3 0.4 0.3 1.8E-11 31500 6.7E-08
2 Degradation of plastic encapsulation
- Discoloration- Carbonization- Polymer degradation at high temperature
- Gradual decrease of optical output
- Excessive drift requires unscheduled calibration- Delayed medical test results
3 0.6 0.7 1.8E-11 31500 2.3E-07
3 Degradation of ITO layer
- Loss of Oxygen from ITO- De-adhesion
- Decrease of optical output- Non-uniform light emission
- Unscheduled module replacement- Delayed medical test results
4 0.3 0.1 1.8E-11 31500 1.7E-08
4 Packaging failure (Bond Wires)
- Electro-migration of bond wires- Burnout due to excessive current- Void formation at the solder metal stem- Reaction of solder metal with package electrodes
- Abrupt LED failure
- Unscheduled module replacement- Delayed medical test results
4 0.9 0.1 1.8E-11 31500 5.0E-08
5 Degradation of active layer
- Dislocation growth- Metal diffusion in AlGaInP- Heating effects of AlGaInP active region resulting in enhanced current injection
- Gradual decrease of optical output
- Excessive drift requires unscheduled calibration- Delayed medical test results
4 0.6 0.6 1.8E-11 31500 2.0E-07
6 Degradation of P-N metal contacts
- Interdiffusion - Change in IV characteristics
- Design will accommodate minor changes in IV characteristics
5 0.4 0.2 1.8E-11 31500 4.5E-08
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Conclusions
Performed Initial FMECA Analysis of LED for Medical Application Accelerated Life Testing (ALT) of AlGaInP LEDs Logarithmic degradation rate modeling Used Arrhenius Model for High Temperature Aging Used Inverse Power Law Model for High Current Aging Optical vs. Vf performance and Spectrum change during ALT Regression Analysis of Prior Publ. Data for AlGaInP / GaN LEDs Comparison of Prior published and Accelerated Life test data. Performed Residual FMECA Analysis after Accelerated Life Test Approach to verify LED suitability for Medical diagnostic appl.
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